Cutting elements, as for example cutting elements used in rock bits or other cutting tools, typically have a body (i.e., a substrate), which has an interface face. An ultra hard material layer is bonded to the interface surface of the body by a sintering process to form a cutting layer, i.e., the layer of the cutting element that is used for cutting. The substrate is generally made from tungsten carbide-cobalt (sometimes referred to simply as “cemented tungsten carbide,” “tungsten carbide” “or carbide” ). The ultra hard material layer is a polycrystalline ultra hard material, such as polycrystalline diamond (“PCD”), polycrystalline cubic boron nitride (“PCBN”) or thermally stable product (“TSP”) material such as thermally stable polycrystalline diamond.
Cemented tungsten carbide is formed by carbide particles being dispensed in a cobalt matrix, i.e., tungsten carbide particles are cemented together with cobalt. To form the substrate, tungsten carbide particles and cobalt are mixed together and then heated to solidify. To form a cutting element having an ultra hard material layer such as a PCD or PCBN ultra hard material layer, diamond or cubic boron nitride (“CBN”) crystals are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure (e.g., a niobium enclosure) and subjected to a high temperature and high pressures so that inter-crystalline bonding between the diamond or CBN crystals occurs forming a polycrystalline ultra hard material diamond or CBN layer. Generally, a catalyst or binder material is added to the diamond or CBN particles to assist in inter-crystalline bonding. The process of heating under high pressure is known as sintering. Metals such as cobalt, iron, nickel, manganese and alike and alloys of these metals have been used as a catalyst matrix material for the diamond or CBN.
The cemented tungsten carbide may be formed by mixing tungsten carbide particles with cobalt and then heating to form the substrate. In some instances, the substrate may be fully cured. In other instances, the substrate may be not fully cured, i.e., it may be green. In such case, the substrate may fully cure during the sintering process. In other embodiments, the substrate maybe in powder form and may solidify during the sintering process used to sinter the ultra hard material layer.
TSP is typically formed by “leaching” the cobalt from the diamond lattice structure of polycrystalline diamond. This type of TSP material is sometimes referred to as a “thermally enhanced” material. When formed, polycrystalline diamond comprises individual diamond crystals that are interconnected defining a lattice structure. Cobalt particles are often found within interstitial spaces in the diamond lattice structure. Cobalt has a significantly different coefficient of thermal expansion as compared to diamond, and as such, upon heating of the polycrystalline diamond, the cobalt expands, causing cracking to form in the lattice structure, resulting in the deterioration of the polycrystalline diamond layer. By removing, i.e., by leaching, the cobalt from the diamond lattice structure, the polycrystalline diamond layer becomes more heat resistant. In another exemplary embodiment, TSP material is formed by forming polycrystalline diamond with a thermally compatible silicon carbide binder instead of cobalt. “TSP” as used herein refers to either of the aforementioned types of TSP materials.
Prior art interface surfaces on substrates have been formed having a plurality of projecting spaced apart concentric annular bands. Tensile stress regions are formed on the upper surfaces of the bands, whereas compressive stress regions are formed on the valleys between such bands. Consequently, when a crack begins to grow it may grow along the entire annular upper surface of the annular band where it is exposed to compressive stresses, or may grow along the entire annular valley between the projections leading to the early failure of the cutting element. In other prior art cutting element substrate interfaces incorporating spaced apart projections 62, the projections have relative flat upper surfaces or non-planar upper surface due a plurality of shallow depressions as shown in FIG. 9. Applicants believe that such upper surfaces allow a crack to grow and gain momentum and thus become critical.
Common problems that plague cutting elements are chipping, spalling, partial fracturing, cracking and/or exfoliation of the ultra hard material layer. Typically, these problems are caused by cracking on the interface between the ultra hard material layer and the substrate and by the propagation of the crack across the interface surface. These problems result in the early failure of the ultra hard material layer and thus, in a shorter operating life for the cutting element. Accordingly, there is a need for a cutting element having an ultra hard material layer with improved cracking, chipping, fracturing and exfoliating characteristics, and thereby having an enhanced operating life.